ARM: at91: move pit timer to common clk framework

Use device tree to get the source clock of the PIT (Periodic Interval Timer).
If the clock is not found in device tree (or dt is not enabled) we'll try to
get it using clk_lookup definitions.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>